Recently, several new broadcasting media or sources have been into services, or on planning for services at a near future. For example, a so-called Broadcast Satellite broadcasting (referred to as BS broadcasting, hereinafter) has been serviced for several years. While a so-called Communication Satellite broadcasting (referred to as CS broadcasting, hereinafter) has recently laied for service. In addition, a so-called CATV system has been in services for long years. These new broadcasting media generally service their broadcasting signals at different frequency bands with each other.
Referring now to FIGS. 1 and 2, a conventional tuning circuit for receiving such a BS broadcasting signal will be briefly described. FIG. 1 is a block diagram showing the conventional tuning circuit. FIG. 2 shows a typical example of the voltage controlled oscillator (referred to as VCO, hereinafter) comprised in the tuning circuit.
In FIG. 1, an input terminal 10 is provided for receiving a prescribed RF signal. The RF signal is converted from a BS broadcasting signal which is received by an antennna system (not shown), so that the BS broadcasting signal is converted to the prescribed RF signal by a converter in the antennna system. The RF signal on the input terminal 10 is then applied to a first RF amplifier 12 and wherein the RF signal is amplified to a suitable level. The RF signal amplified in the first RF amplifier 12 is then applied to an attenuator 14 so that the RF signal is attenuated for reducing noise components. The RF signal attenuated in the attenuator 14 is then applied into a second RF amplifier 16 wherein the RF signal is again amplified to restore the level of the RF signal. The RF signal amplified in the second RF amplifier 16 is then applied to a band pass filter (referred to as BPF, hereinafter) 18. The BPF 18 passes therethrough a prescribed frequency band of the RF signal for removing undesired frequency components out of the frequency band. The RF signal passing through the BPF 18 is then applied to a mixer 20. The mixer 20 receives a local oscillation signal (referred to as LO signal, hereinafter), other than the RF signal. The LO signal is generated by a VCO 22, and then applied to the mixer 20 through a buffer amplifier 24. In the mixer 20, the RF signal is frequency converted to an intermediate frequency signal (referred to as IF signal, hereinafter) in the up-heterodyne fashion using the LO signal.
The IF signal output from the mixer 20 is applied to a first IF amplifier 26 wherein the IF signal is amplified to a suitable level. The IF signal amplified in the first IF amplifier 26 is then applied to an IF filter 28. The IF filter 28 passes therethrough a prescribed frequency band of the IF signal for removing undesired frequency components out of the IF frequency band. The IF signal passing through the IF filter 28 is then applied to a second IF amplifier 30 wherein the IF signal is again amplified to another suitable level. The IF signal amplified in the second IF amplifier 30 is then applied to a demodulator 32. The demodulator 32 demodulates a baseband signal from the IF signal, so that the baseband signal is output from the tuning circuit through an output terminal 34.
In such an arrangement of the conventional tuning circuit, the RF signal applied to one input of the mixer 20 has a frequency band of about 1035 MHz through 1332 MHz, while the mixer 20 is provided to output the IF signal with a prescribed frequency of 402.78 MHz. Thus, the LO signal applied to the other input of the mixer 20 from the VCO 22 is varied its frequency in the range of about 1478 MHz through 1735 MHz.
One example of the VCO 22 is illustrated in FIG. 2. As shown in FIG. 2, a series circuit of an inductor 40 and a variable capacitor 42 is coupled between the base of a transistor 44 and a reference potential source, e.g., a ground circuit. The transistor 44 is coupled to three capacitors 46, 48 and 50. The capacitor 46 is coupled across the collector and the emitter of the transistor 44. The capacitor 48 is coupled between the collector of the transistor 44 and the reference potential source. The capacitor 50 is coupled across the emitter and the base of the transistor 44. In this arrangement of the VCO 22, the oscillation frequency of the VCO 22, i.e, the LO signal is changed by varying the capacitance of the variable capacitor 42.
The frequency F of the LO signal obtained by the VCO 22 is given by the following Equation (1). EQU F=.alpha..multidot.(L.multidot.C).sup.-1/2 ( 1)
wherein L and C represent the inductance of the inductor 40 and the capacitance of the variable capacitor 42, respectively. In such arrangement of the VCO 22, the inductance L of the inductor 40 is set at a constant value while the capacitance C of the variable capacitor 42 is made variable. Therefore the variable range of the oscillation frequency F depends on a capacitance varying ratio of the capacitance C. Generally, the variable capacitor 42 used in VCOs oscillating around 1 GHz has a variable range of the capacitance C of about 0.7 pF through 4.0 pF. Thus the capacitance varying ratio of the capacitance C is given by (4/0.7).sup.1/2 .congruent.2.4.
In a practical circuit construction, the oscillation frequency F is influenced by many factors such as stray capacitances accompanied with the circuit construction. Thus the variable range of the frequency F is limited to a relatively smaller range of about 1000 MHz through 2400 MHz with respect to the capacitance varying ratio of 2.4.
This frequency range of 1000 MHz through 2400 MHz is satisfactory for the tuning circuits adapted for receiving only the BS broadcasting signals. However, if the uner is attempted to be able to receive the CS broadcasting signals as well as the BS broadcasting signals, the tuning circuit will be accompanied with some difficulties. This is because the BS and CS broadcasting media serve their broadcasting signals at different frequency bands with each other, as described before. Thus the conventional tuning circuit can receive a limited frequency range but not receive an entire frequency band extending over both frequency ranges of the BS and CS broadcasting signals. Therefore, in the conventional tuning circuit adapted for receiving multi-channel signals it was required a very complicated arrangement. For example, a plurality of inductors were provided for selectively defining different oscillation frequency ranges, or a plurality of VCOs were provided for selectively supplying different oscillation frequency ranges. In the former type the conventional tuning circuit failed to oscillate a stable frequency because of influences due to capacitances accompanied with a switch for selecting the inductors. In the latter type the conventional tuning circuit had a cost problem and a space problem for accomodating many VCOs.
Therefore it is desired to provide a novel tuning circuit which is able to stably receive broadcasting signals over a very broad band channel at a simple construction.